This invention relates in general to rotating seals and in particular to a new and useful shaft seal.
The invention relates particularly to a fluid-locked shaft seal having a seal ring which rotates with the shaft and is provided with flow channels, a fixed packing ring adapted for flexible axial movement and sealingly connected with the housing, and a slide ring which is arranged between the rotating seal ring and the fixed packing ring and rotates at approximately half the speed of rotation of the shaft.
Shaft seals of the above named kind are used for packing between the rotating shaft and the housing of fluid energy machines for the compression and expansion of gases.
In the shaft seal according to British Pat. No. 1,269,285, it is provided to generate in the seal ring revolving at the shaft speed, in radial channels arranged therein and by pumping action, a flow of the cooling oil which is deflected in axial directions, conveyed through axial bores in the slide ring and returned into the oil cycle through further bores in stationary parts. The slide ring is to revolve approximately at half the speed of the shaft. Measures required for this have not been disclosed.
The speed of rotation of the freely rotating slide ring is determined by the torques acting on it. The latter are determined essentially by the friction moments generated at the two end faces.
In the sliding ring seal here considered, the coefficient of friction depends little on the speed of rotation, so that the drive moment M.sub.I transmitted from the rotating seal ring to the sliding ring by friction is generally smaller at all speeds n.sub.G of the sliding ring than the braking moment. The braking moment consists of the friction moment between sliding ring and non-rotating packing ring M.sub.II and that of the wall friction M.sub.M of the sliding ring (externally in liquid, internally in gas), which increases with increasing speed (FIG. 7 and 8). Through different diameters and coefficients of friction of the two axial slide surfaces of the sliding ring, variations of the sketched curve forms are possible within limits, but the uncertainty of the definition of a unique operational speed of rotation n.sub.G * of the sliding ring remains.
The liquid stream issuing with twist out of the seal ring 2 rotating at full speed causes no turbine torque in the freely rotating sliding ring 12 at shockfree inflow to the axial bores 19, since then the velocity triangles at the entrance and exit of these bores are the same.
If the inflow in the relative system is not shockfree, a torque may be exerted, although it is questionable what effect this will still have due to the flow losses at the sliding ring 12 caused by this faulty approach flow and whereby one half the speed of rotation of the sliding ring 12 relative to the shaft is to be brought about. Evidently the sliding ring 12 is not conceived as a turbine wheel.
If the sliding ring does not rotate or does so only at a low, not clearly defined speed, the now maximum relative sliding velocity between rotating seal ring and sliding ring generates locally a considerable friction heat. An adverse effect, especially at high speeds of rotation, is that of the additional flow losses caused by the faulty approach with regard to an additional temperature increase, which must be avoided. Further flow losses occur due to the fact that through the shown flow conduction after issuance from the sliding ring an extensive whirling of the flow velocity (sic) issuing from the sliding ring is brought about, so that further losses come about through the required transformation of static pressure energy into velocity at the entrance to the pump wheel.
Such a shaft seal is suitable, therefore, for low to medium sliding velocities. At high sliding velocities overheating may occur due to insufficient removal of heat, leading to premature wear of the sliding ring. A torque, unambiguously definable for the speed of the freely rotating sliding ring at smallest possible flow losses is not obtainable thereby.